it's shocking how quick the price of wood adds up
I already own something similar. It's huge. It's heavy. I have to store it at work in the warehouse, the only place with enough real estate to store it. It's a pain to sling it back and forth
These are the reasons I was thinking it might be best to spend a little more for a compact, portable, professionally built lift (something like the one pictured with the Rally car). There are several versions of this concept on the market, naturally prices vary widely so shop around. A few images from Google:
Some of the portable lifts shown above remind me of a large car jack that my dad had when I was a little boy. It had a hydraulic ram (like a big bottle jack) lifting a pair of arms (like short fork lift arms). It was placed under the bumper (front or rear) of the car to lift that end of the vehicle about 2.5 - 3 feet off the ground. And with two of them the entire car could be suspended in the air from it's bumpers...naturally bumpers were much more robust in those days. Although it no longer works, it is still sitting in a storage shed on our property. I've often thought about seeing if it could be rebuilt and configured to work with today's vehicles somehow.
If you own your property, intend to stay there awhile, and want something more permanent, then for not much more money you can buy a 2-post lift. Requires hard mounting into the foundation:
I fully understand budgetary concerns or other limitations. But building a large, heavy, wooden contraption that may not be safe and might cost more than you think, could be poor economy.
Perhaps another approach might be building tall stands to place the car on. First use a engine hoist ("cherry picker") to lift one end of the car high enough to place it on the stand(s). Then lift and support the other end likewise. The process is something like this:
The stand shown above is really TOO tall, but gives the idea. Or you can buy a pair (or two pair) of tall stands:
But I'll warn you, these tall stands are expensive. You could buy one of the portable lifts for the same money.
Hope all this gets your thoughts stirring.